## De Rham cohomology and conductors of curves.(English)Zbl 0632.14018

This paper shows that the conductor of an arithmetic surface (a two- dimensional scheme flat and proper over $${\mathbb{Z}})$$ is the Euler characteristic of the torsion of the de Rham complex. The conductor appears in the (conjectured) functional equation for the Hasse-Weil zeta- function of the scheme and has an expression as the product of local terms which in turn can be expressed in terms of local contributions to the étale cohomology of the surface. The desired formula therefore gives a relation between local contributions to the de Rham cohomology and the étale cohomology of the scheme. It is proved by the intermediary of the Riemann-Roch formula and an expression of a local Chern class in terms of the local contribution to the Euler-Poincaré formula. In the course of the argument an expression of the conductor as the square root of the discriminant of the cup product of the de Rham complex truncated at degree 1 is obtained.
Reviewer: T.Ekedahl

### MSC:

 14F40 de Rham cohomology and algebraic geometry 14G10 Zeta functions and related questions in algebraic geometry (e.g., Birch-Swinnerton-Dyer conjecture) 14J20 Arithmetic ground fields for surfaces or higher-dimensional varieties 14G25 Global ground fields in algebraic geometry
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### References:

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